Under the current tendency of office automation fashion, the copying machine, the printer and the scanning machine, etc. have widely been used for document-processing. In order to elevate the operation efficiency of the scanning machine or the copying machine, an automatic document feeder has been designed and developed to be attached to the aforesaid machines, i.e., a plurality of sheet papers can be fed into the machine continuously and automatically instead of being fed one sheet at a time by a person. The transmission structure of the feeder is shown in FIG. 1, which comprises a scanning zone one-way gear train 10, a feeding zone one-way gear train 11, an output zone one-way gear train 12, and a driving gear 13. The transmission theory is based on a concept that the driving direction of the feeding zone one-way gear train 10 is contrary to that of the scanning zone one-way gear train 11 and the output zone one-way gear train 12. When the driving gear 13 rotates to feed a sheet of paper (for example, to rotate clockwise), the feeding zone one-way gear train 10 moves towards a correct direction to drive a transmission shaft to rotate so as to have the first sheet 15 of document fed to the front of a scanning line 14 (as shown in FIG.2); then, as soon as the first sheet 15 of document is fed to the stand-by position before the scanning line, the driving gear 13 starts to rotate for scanning and output process (i.e., rotating counter-clockwise); at the same time, the feeding zone one-way gear train 10 is unable to drive the transmission shaft to rotate, while the scanning zone one-way gear train 11 and the output zone one-way gear train 12 will drive the corresponding transmission shaft to rotate and to cause the first sheet 15 to pass through the scanning line 14 (as shown in FIG. 3) and to move out of the output zone. As soon as the first sheet 15 moves out completely, the driving gear 13 will restore to the feeding-step, rotation (i.e., rotating clockwise) so as to process a second sheet 16 of document (as shown in FIG. 4), and then the same procedures are repeated automatically to fulfill a document feeding operation.
As pointed out for the aforesaid conventional paper-feeding structure, the feeding distance D1 between two sheets of document is rather large, and therefore the feeding speed can not be elevated to meet the requirement of high speed and high efficiency, i.e., becoming a bottleneck in terms of efficiency.